The United States informed France ahead of its missile strikes on Syrian military positions, France's Foreign Minister Jean-Marc Ayrault said on Friday.

"I was told by (U.S. Secretary of State) Rex Tillerson during the night," Ayrault told Reuters and France Info radio in the Mauritanian capital Nouakchott where he is on a diplomatic visit.

"Use of chemical weapons is appalling and should be punished because it is a war crime," he said, adding that Russia and Iran needed to understand that supporting Syrian President Bashar al-Assad made no sense, and that France was not seeking a confrontation with those two countries.